Getting pregnant (conception) happens when a man's sperm fertilises a woman's egg. For some women this happens quickly, but for others it can take longer.

Out of every 100 couples trying for a baby, 80 to 90 will get pregnant within 1 year. The rest will take longer, or may need help to conceive.

To understand conception and pregnancy, it helps to know about the male and female sexual organs, and to understand how a woman's monthly menstrual cycle and periods work.

The menstrual cycle is counted from the first day of a woman's period (day 1). Some time after her period she will ovulate, and then around 12-16 days after this she'll have her next period. The average cycle takes 28 days, but shorter or longer cycles are normal.

The best time to get pregnant

You're most likely to get pregnant if you have sex within a day or so of ovulation (releasing an egg from the ovary). This is usually about 14 days after the first day of your last period, if your cycle is around 28 days long.

An egg lives for about 12-24 hours after being released. For pregnancy to happen, the egg must be fertilised by a sperm within this time.

Sperm can live for up to 7 days inside a woman's body. So if you've had sex in the days before ovulation, the sperm will have had time to travel up the fallopian tubes to "wait" for the egg to be released.

It's difficult to know exactly when ovulation happens, unless you are practising natural family planning, or fertility awareness.

Have frequent sex

If you want to get pregnant, having sex every 2 to 3 days throughout the month will give you the best chance.

You don't need to time having sex only around ovulation.

The male sexual organs

The penis: this is made of sponge-like erectile tissue that becomes hard when filled with blood.

Testes: men have two testes (testicles), which are glands where sperm are made and stored.

Scrotum: this is a bag of skin outside the body beneath the penis. It contains the testes and helps to keep them at a constant temperature just below body temperature. When it's warm, the scrotum hangs down, away from the body, to help keep the testes cool. When it's cold, the scrotum draws up, closer to the body for warmth.

Vas deferens: these are two tubes that carry sperm from the testes to the prostate and other glands.

Prostate gland: this gland produces secretions that are ejaculated with the sperm.

Urethra: this is a tube that runs down the length of the penis from the bladder, through the prostate gland to an opening at the tip of the penis. Sperm travel down this tube to be ejaculated.

The female sexual organs

A woman's reproductive system is made up of both external and internal organs. These are found in the pelvic area, the part of the body below the belly button.

The external organs are known as the vulva. This includes the opening of the vagina, the inner and outer lips (labia) and the clitoris.

The woman's internal organs are made up of:

The pelvis: this is the bony structure around the hip area, which the baby will pass through when he or she is born.

Womb or uterus: the womb is about the size and shape of a small, upside-down pear. It's made of muscle and grows in size as the baby grows inside it.

Fallopian tubes: these lead from the ovaries to the womb. Eggs are released from the ovaries into the fallopian tubes each month, and this is where fertilisation takes place.

Ovaries: there are 2 ovaries, each about the size of an almond; they produce the eggs, or ova.

Cervix: this is the neck of the womb. It's normally almost closed, with just a small opening through which blood passes during the monthly period. During labour, the cervix dilates (opens) to let the baby move from the uterus into the vagina.

Vagina: the vagina is a tube about 3 inches (8cm) long, which leads from the cervix down to the vulva, where it opens between the legs. The vagina is very elastic, so it can easily stretch around a man's penis, or around a baby during labour.

The woman's monthly cycle

The video below at the bottom of the page shows what happens during the menstrual cycle.

Ovulation occurs each month when an egg is released from one of the ovaries. Occasionally, more than one egg is released, usually within 24 hours of the first egg.

At the same time, the lining of the womb begins to thicken and the mucus in the cervix becomes thinner, so that sperm can swim through it more easily.

The egg begins to travel slowly down the fallopian tube. The egg may be fertilised here if there is sperm in the fallopian tube. The lining of the womb is now thick enough for the egg to be implanted in it after it has been fertilised.

If the egg is not fertilised, it passes out of the body during the woman's monthly period, along with the lining of the womb. The egg is so small that it cannot be seen.

Pregnancy hormones

Hormones are chemicals that circulate in the blood of both men and women. They carry messages to different parts of the body, regulating certain activities and causing certain changes to take place.

The female hormones, which include oestrogen and progesterone, control many of the events of a woman's monthly cycle, such as the release of the egg from the ovary and the thickening of the womb lining.

During pregnancy, your hormone levels change. As soon as you have conceived, the amount of oestrogen and progesterone in your blood increases. This causes the womb lining to build up, the blood supply to your womb and breasts to increase, and the muscles of your womb to relax to make room for the growing baby.

The increased hormone levels can affect how you feel. You may have mood swings, feel tearful or be easily irritated. For a while, you may feel that you can't control your emotions, but these symptoms should ease after the first 3 months of your pregnancy.

Will it be a boy or a girl?

Both the man's sperm and the woman's egg play a part in determining the gender of a baby. Every normal human cell contains 46 chromosomes (23 pairs), except for the male sperm and female eggs. They contain 23 chromosomes each.

When a sperm fertilises an egg, the 23 chromosomes from the father join with the 23 from the mother, making 46 in total.

X and Y chromosomes

Chromosomes are tiny threadlike structures that each carry about 2,000 genes. Genes determine a baby's inherited characteristics, such as hair and eye colour, blood group, height and build.

A fertilised egg contains 1 sex chromosome from its mother and 1 from its father. The sex chromosome from the mother's egg is always the same and is known as the X chromosome, but the sex chromosome from the father's sperm may be an X or a Y chromosome.

If the egg is fertilised by a sperm containing an X chromosome, the baby will be a girl (XX). If the sperm contains a Y chromosome, the baby will be a boy (XY).